Heat pump systems provide heating and cooling to enclosed spaces, such as buildings and homes. A heat pump system typically comprises an indoor unit that includes a heat exchanger coil and a fan and an outdoor unit that includes a heat exchanger coil, a fan, and a compressor. The heat pump system also typically includes multiple valves, such as an expansion valve and a reversing valve. The indoor unit and outdoor unit are typically linked by tubing that includes a refrigerant fluid. The reversing valve changes the direction in which the refrigerant fluid flows depending on the mode of operation so that the heat pump system can operate in a cooling mode to provide cooling to an enclosed space and can operate in a heating mode to provide heating to an enclosed space. In the cooling mode, the outdoor heat exchanger transfers heat from the refrigerant to the air circulating through the outdoor unit. In contrast, in the heating mode, the outdoor heat exchanger transfers heat from the air circulating through the outdoor unit to the refrigerant fluid.
The heating capacity of the heating mode becomes more limited as the outdoor temperature decreases because there is less heat available in the circulating air that can be transferred to the refrigerant. In climates with extremely cold temperatures, the heat pump may not have sufficient heating capacity to warm the enclosed space. In such cases, a backup heating system, such as an indoor electric heating system may be required. However, backup heating systems, such as an indoor electric heating system, add costs and may not be as efficient.
Therefore, in view of the limitations in existing heat pump systems, there is a need for a heat pump system that provides improved heating capacity in cold climates. There is a further need for a heat pump system that provides additional heating capacity efficiently and without the need to install a backup indoor heating system.